Headphones? This is dangerous. With the built-in speaker of the headphones a thing of the past when cycling. This increases safety, because you get with what's going on around you.
Disadvantage of volume control: because of duplicate key assignment (. = Board Back loudspeakers Quiet when you press longer function as on the mobile phone Sony Ericsson K800i Mp3 player) you're on quickly to a song.
In addition, the light is not exactly a revelation. Battery has approximately every 3 - 7 Tage (5h continuous operation) to be recharged.
Great idea, moderately implemented.
To have in the store and at Trekstor plus.de also to price by the manufacturer (under 20 euros), so definitely worth a try.